The present invention relates to a heterojunction bipolar transistor that is a semiconductor device of a III-V compound, a semiconductor device employing the same and a heterojunction bipolar transistor fabricating method.
Conventionally, as an AlGaAs/GaAs heterojunction bipolar transistor (HBT), there has been a structure as shown in FIG. 8. In FIG. 8, there are shown an Au/Ge/Ni emitter ohmic electrode 1, an n+-type GaAs cap layer 2 (100 nm in thickness, donor concentration n=5xc3x971018 cmxe2x88x923 (n representing the donor concentration hereinafter)) and an n-type Al1xe2x88x92yGayAs crystal mixture ratio graded cap layer 3 (20 nm in thickness, n=5xc3x971017 cmxe2x88x923, y=0.35xe2x86x920.0 (y=0.35 on the substrate side and y=0 on the surface side)). There are also shown an n-type Al0.35Ga0.65As ballast layer 4 (200 nm in thickness, n=5xc3x971016 cmxe2x88x923), an n-type Al0.3Ga0.7As emitter layer 5 (10 nm in thickness, n=5xc3x971017 cmxe2x88x923), a p+-type GaAs base layer 6 (80 nm in thickness, acceptor concentration p=2xc3x971019 cm xe2x88x923 (p representing the acceptor concentration hereinafter)) and a Ti/Pt/Au base ohmic electrode 7. There are further shown an n-type GaAs collector layer (700 nm in thickness, n=2xc3x971016 cmxe2x88x923), an n+-type GaAs sub-collector layer 9 (500 nm in thickness, n=5xc3x971018 cmxe2x88x923), an Au/Ge/Ni collector ohmic electrode 10 and a semi-insulating GaAs substrate 11.
In connection with the AlGaAs/GaAs HBT having the aforementioned construction, it is well known that integrating the ballast layer having a crystal mixture ratio x of 0.15xe2x89xa6xxe2x89xa60.4 increases the temperature coefficient of the ballast resistance and is effective for the uniformity of current and the stability of temperature through the restraint of thermal runaway within a wide range of temperature when the crystal mixture ratio x is adjusted(prior art reference of Japanese Patent Laid-Open Publication No. HEI 6-349847). The HBT having an AlGaAs ballast layer as described above has an emitter layer made of only AlGaAs that has a doping concentration and an Al crystal mixture ratio different from those of the ballast layer.
However, the aforementioned conventional AlGaAs/GaAs HBT has had the following problems. That is, during operation at a high temperature and a high current, the AlGaAs/GaAs HBT having the aforementioned AlGaAs ballast layer causes a serious problem that the resistance of the ballast layer is disadvantageously reduced due to a high junction temperature.
The reduction in ballast resistance due to the increase in junction temperature often reduces the effect of the ballast resistance as follows. In detail, FIG. 9 shows a graph of collector current density Jc to base-to-emitter voltage Vbe characteristics (Jc-Vbe characteristics). According to these Jc-Vbe characteristics, the junction temperature increases much when a collector-to-emitter voltage Vce is high, and this causes a curve of a negative slope. This fact indicates the instability of the aforementioned HBT and causes a non-uniformity in terms of current. The above-mentioned negative slope is ascribed to a modulation of conductivity (reduction in resistance) of the ballast resistance layer due to hole injection.
FIG. 10 schematically shows the band structure of the aforementioned HBT. The band offset energy xcex94Ev of the valence band between AlGaAs and GaAs is about 36% of a difference xcex94Eg between their band gap energies, and the remaining 64% is the band offset energy xcex94Ec of the conduction band between them. As it is shown in FIG. 11, the band offset energy xcex94Ev is defined as an energy barrier in the valence band generated at the interface of two different semiconductor materials. When one material has electron affinity "khgr"1 and band gap energy Eg1 and the other material has electron affinity "khgr"2 and band gap energy Eg2, the energy barrier xcex94Ev is expressed by:
xe2x80x83xcex94Ev=("khgr"2+Eg2)xe2x88x92("khgr"1+Eg1).
The band offset energy xcex94Ev(x) with respect to AlGaAs can be expressed by xcex94Ev(x)=0.449x(eV) (0 less than x less than 0.45). Therefore, the band offset energy xcex94Ev in the valence band with respect to typical AlGaAs/GaAs of a crystal mixture ratio x=0.3 becomes about 135 meV (=0.449xc3x970.3xc3x97103 meV). As a result, hole injection from the base layer to the emitter layer occurs, as a consequence of which the current gain hFE of the AlGaAs/GaAs HBT is significantly reduced at high temperature. The temperature dependency of hole injection at a constant collector current is given by Jpxcx9cexp(xe2x88x92xcex94Ev/kT). In this case, Jp represents the Hall current density, k represents the Boltzmann""s constant and T represents the absolute temperature. Each hole injected from the base layer into the emitter layer diffuses toward the ballast layer, and the movement of electrons from the GaAs cap layer to the ballast layer occurs in order to satisfy space charge neutralization condition. Under the conditions of high current and high temperature, the hole density of the ballast layer is equivalent to the dope concentration of the ballast layer. As a result, the aforementioned electron movement causes a reduction in the resistance of the ballast layer. FIG. 10 schematically shows this phenomenon.
The hole density in the ballast layer at high current and high temperature can be calculated from the aforementioned temperature dependency of the current gain hFE of the HBT and the mobility of an electron and a hole as follows:
p≈NDxcexce/(hFE(T)xcexch)
where p represents the hole density in the ballast layer, ND represents the doping concentration of the donor in the ballast layer, hFE(T) represents the temperature-dependent current gain, and xcexce and xcexch are the mobility of an electron and a hole respectively. According to the above expression, assuming that the doping concentration ND of the donor in the ballast layer is ND=5xc3x971016cmxe2x88x923 and the current gain hFE is hFE=30, then the hole density to be injected into the ballast layer is 2.2xc3x971016 cmxe2x88x923 in the typical AlGaAs/GaAs HBT where the electron mobility and hole mobility are 4000 cm2/Vsec and 300 cm2/Vsec, respectively. This hole density is comparable to the doping concentration ND, and this indicates that a considerable degree of conductivity modulation (reduction in resistance) of the ballast layer occurs. This conductivity modulation exhibits a similar characteristic on an AlGaAs ballast layer having a crystal mixture ratio x in the range of 0.15xe2x89xa6xxe2x89xa60.4 but also an AlGaAs ballast layer in the range of 0xe2x89xa6xxe2x89xa60.45. For a given ballast resistance, the smaller the crystal mixture ratio x becomes, the greater the electron mobility becomes, so that the donor concentration in the ballast layer becomes low. Therefore, the hole injection becomes a more serious problem. In contrast to this, the ballast layer generally comes to have a high density when the crystal mixture ratio x exceeds 0.45, and the hole injection comes to exert scarce influence. However, the ballast layer originally has a poor performance when the crystal mixture ratio x is not smaller than 0.45.
Accordingly, an object of the present invention is to provide an HBT capable of restraining the conductivity modulation of the ballast layer that is the cause of a deterioration in temperature characteristics by preventing hole injection from the base layer into the emitter layer.
In order to achieve the above-mentioned object, the present invention provides a heterojunction bipolar transistor having a stack comprised of a base layer, an emitter layer and a ballast layer made of AlGaAs, wherein the emitter layer is comprised of a single layer or a multiplicity of layers, and at least one of which is comprised of a material that prevents hole injection from the base layer into the ballast layer.
According to the construction of the present invention, the hole injection from the base layer into the ballast layer is prevented by the emitter layer, and therefore, the reduction in resistance of the AlGaAs ballast layer is prevented. Therefore, uniformity of current and temperature can be obtained even at a high temperature and a high current density without hindering the effective ballast effect by virtue of the resistance of the high temperature coefficient of the AlGaAs ballast layer.
The present invention also provides a heterojunction bipolar transistor having a stack comprised of a base layer, an emitter layer and a ballast layer made of AlGaAs, wherein the emitter layer is comprised of a single layer or a multiplicity of layers, at least one layer of which has a value xcex94Ev/xcex94Eg, which is a ratio of the valence band offset energy xcex94Ev between the one layer and the base layer to the band gap energy difference xcex94Eg between the one layer and the base layer, being greater than 0.36.
According to the construction of the present invention, the energy barrier between said one layer of the emitter layer and the base layer becomes greater than the energy barrier between the conventional AlGaAs emitter layer and the GaAs base. Thus, the hole injection from the base layer into the ballast layer is prevented by the emitter layer, and therefore, the reduction in resistance of the AlGaAs ballast layer is prevented. Therefore, uniformity of current and temperature can be obtained even at a high temperature and a high current density without hindering the effective ballast effect by virtue of the resistance of the high temperature coefficient of the AlGaAs ballast layer.
The present invention also provides a heterojunction bipolar transistor having a stack comprised of a base layer, an emitter layer and a ballast layer made of AlGaAs, wherein the emitter layer is comprised of a single layer or a multiplicity of layers, at least one layer of which has a valence band offset energy xcex94Ev between the one layer and the base layer being greater than 135 meV.
According to the construction of the present invention, similarly to the aforementioned construction, the energy barrier between said one layer of the emitter layer and the base layer becomes greater than the energy barrier between the conventional AlGaAs emitter layer and the GaAs base. Thus, the hole injection from the base layer into the ballast layer is prevented by the emitter layer, and therefore, the reduction in resistance of the AlGaAs ballast layer is prevented. Therefore, uniformity of current and temperature can be obtained even at a high temperature and a high current density without hindering the effective ballast effect by virtue of the resistance of the high temperature coefficient of the AlGaAs ballast layer.
In one embodiment, said one layer of the emitter layer is made of at least one of InGaP, InAlP, InGaAlP, InAlAsP or InGaAlAsP.
According to the construction of this one embodiment, if the base layer is made of a GaAs layer, the value xcex94Ev/xcex94Eg, which is the ratio of the valence band offset energy xcex94Ev between said one layer and the base layer to the band gap energy difference xcex94Eg between said one layer and the base layer, becomes greater than 0.36. In addition to this, the valence band offset energy xcex94Ev between said one layer and the base layer becomes greater than the value of 135 meV. Therefore, uniformity of current and temperature can be obtained even at a high temperature and a high current density without hindering the effective ballast effect by virtue of the resistance of the high temperature coefficient of the AlGaAs ballast layer.
In one embodiment, the emitter layer is comprised of a one layer made of InGaP, InAlP, InGaAlP, InAlAsP or InGaAlAsP.
According to the construction of this one embodiment, the whole body of the emitter layer can be operated as a barrier layer against holes that tend to move from the base layer into the ballast layer. Therefore, uniformity of current and temperature can be obtained even at a high temperature and a high current density without hindering the effective ballast effect by virtue of the resistance of the high temperature coefficient of the AlGaAs ballast layer.
In one embodiment, the ballast layer is comprised of an n-type semiconductor having a donor concentration lower than that of the emitter layer, and at least part of the ballast layer is an AlxGa1xe2x88x92xAs layer having an Al crystal mixture ratio x in a range of 0 less than xxe2x89xa60.45.
According to the construction of this one embodiment, by comparison with the ballast layer constructed of GaAs, the space charge modulation by the electrons which move from the emitter layer to the ballast layer scarcely occurs, because the ballast layer made of AlGaAs having a smaller electron affinity. Therefore, the resistance value of the ballast resistance layer does not change even at a high temperature and a high current density, allowing uniformity of current and temperature to be obtained.
In one embodiment, at least part of the ballast layer is an AlxGa1xe2x88x92xAs layer having an Al crystal mixture ratio x in a range of 0.12 less than xxe2x89xa60.4.
According to the construction of this one embodiment, the space charge modulation by the electrons which moves from the emitter layer to the ballast layer scarcely occurs because the crystal mixture ratio x is set so that 0.15xe2x89xa6x, and the temperature coefficient of the resistance value of the ballast layer is great because the crystal mixture ratio x is set so that xxe2x89xa60.4. By the combination of them, uniformity of current and temperature can be obtained in a wide range of high temperature.
The present invention also provides a heterojunction bipolar transistor having a stack comprised of a base layer, an emitter layer and a ballast layer, wherein the emitter layer is comprised of a multiplicity of layers, and one layer of the emitter layer that adjoins the base layer is made of InGaP, InAlP, InGaAlP, InAlAsP or InGaAlAsP.
According to the construction of the present invention, the one layer of the emitter layer that adjoins the base layer is made of InGaP, InAlP, InGaAlP, InAlAsP or InGaAlAsP. With this arrangement, the hole injection from the base layer into the ballast layer is reliably prevented by said one layer even at a high temperature and a high current density. Then, this provides a synergistic effect with the effective ballast effect by virtue of the resistance of the high temperature coefficient of the AlGaAs ballast layer. Therefore, reliability of the HBT is improved.
In one embodiment, the emitter layer has a two-layer structure comprised of an AlGaAs layer and an InGaP layer.
The present invention further provides a heterojunction bipolar transistor having a stack comprised of a base layer, an emitter layer and a ballast layer, wherein the emitter layer is comprised of a multiplicity of layers, and one layer of the emitter layer that adjoins the ballast layer is made of InGaP, InAlP, InGaAlP, InAlAsP or InGaAlAsP.
According to the construction of the present invention, the hole injection from the base layer into the ballast layer is prevented by said one layer of the emitter layer even at a high temperature and a high current density, and the base current is increased by the recombination of electron and hole in the emitter region up to the aforementioned one layer. As a result, a different type of ballast effect of reducing the collector current is generated, and this allows the obtainment of a further improved uniformity of current and temperature even at a high temperature and a high current density.
In one embodiment, the emitter layer has a two-layer structure comprised of an AlGaAs layer and an InGaP layer.
According to the construction of the present invention, the hole injection from the base layer into the ballast layer is prevented by the above InGaP layer of the emitter layer even at a high temperature and a high current density, and the base current is increased by the recombination of electron and hole in the AlGaAs layer of the emitter layer up to the aforementioned InGaP layer. As a result, a different type of ballast effect of reducing the collector current is generated, and this allows the obtainment of a further improved uniformity of current and temperature even at a high temperature and a high current density.
According to the aforementioned constructions, the injection and diffusion of holes from the base layer into the ballast layer can be sufficiently prevented even at a high temperature and a high current density. This can consequently obviate the need for increasing the thickness of the ballast layer and increasing the resistance taking into account the reduction in ballast resistance due to the hole injection at a high temperature and a high current density, and consequently allows the thickness and resistance of the ballast layer to be reduced. As a result, the step difference on the surface of the HBT device becomes small, so that the HBT device which has a low resistance and high performance can be prepared easily.